The Esoteric disc transports (P1/P1X/P1XSE) are the only products with ES-LINK outputs. You would not have two Esoteric disc transports in your system connected to D1/D1X/D1XSE, so that makes one wonder why it has dual ES-LINK input. I suspect Esoteric will release a new device with ES-LINK output, perhaps N1T network transport. So N1T and P1XSE will be connected to the D1XSE. And it makes sense why N01XD/N01XDSE only has one ES-LINK input, because you wouldn't connect N1T to one of them, you'd be connecting the only other thing with ES-LINK.
A similar situation arose when Esoteric released G1X with five outputs, which didn't make much sense at the time as 4 devices was the full set for external clocking. When Esoteric announced T1 it then made sense why G1X has 5 outputs. They don't do this stuff randomly, there must be a new device in the Esoteric skunk works that is not a disc transport which will have an ES-LINK output.